Recently, in a take-up device of a yarn used for a synthetic fiber production process, for example, higher performance has been sought so as to improve production efficiency, to reduce the cost of production and to improve and discriminate yarn quality. A take-up device equipped with a bobbin holder to which a bobbin having an entire length of greater than 1,000 mm is fitted and which can take up a yarn at a high speed of not lower than 6,000 m/min has been accomplished nowadays.
However, a further improvement in productivity and the reduction of the cost of production are extremely important factors for the synthetic fiber industry, and development of a bobbin holder of a so-called "small diameter elongated type", which can at a time grip a large number of small diameter bobbins capable of reducing the bobbin cost, and which can attain high speed take-up, and a take-up device using such a bobbin holder, has been desired.
The bobbin holders used for recent yarn take-up devices have been elongated in accordance with the requirement described above and have been used predominantly in the range of low number of revolutions which is lower than a high-order critical speed (hereinafter merely called the "critical speed (Nc)") which exceeds a low critical speed and which cannot be exceeded because vibration energy is excessively great.
An example of the conventional bobbin holders of such a type is described, for example, in Japanese Patent Laid-Open No. 225268/1990, and its general construction is shown in FIG. 11. Namely, the conventional bobbin holder 10 is fixed to the extreme left end portion of a bobbin holder shaft 14 directly coupled to a motor shaft 12 through a coupling 13, and comprises a rotary cylinder 15, a plurality of flexible rings 16a to 16h (eight rings are shown in the drawing) inserted to the outer periphery of the rotary cylinder, cylindrical spacers 17a to 17g for positioning the flexible rings, a front cover 17 for pushing the flexible rings 16a to 16h in the right-hand direction in the drawing and a push mechanism 20 comprising a coned disc spring 18 and a piston 19, for imparting the push force to the front cover in the right-hand direction in the drawing. Feed ports 12a and 14a for supplying compressed air are disposed in the motor shaft 12 and in the bobbin holder 14, respectively, and the flexible rings 16a to 16h are integrated by bonding a steel ring 22 to both side surfaces of a rubber ring 21 as shown in FIG. 12.
When bobbins 23 are gripped by the bobbin holder 10, the front cover 17 is moved to the right in the drawing by the push force of the coned disc spring (expanding force in the right-hand direction in the drawing) so as to compress each flexible ring 16a, 16h from the direction of both side surfaces. Since the volume of the rubber ring 21 hardly changes in this instance, the rubber ring increases its diameter and undergoes deformation by a volume corresponding to the change of compression from the direction of both side surfaces, and grips the bobbins 23. In other words, the greater the thickness of the rubber ring in the radial direction, the greater becomes the change of the volume with respect to an equal compressive change from the direction of the side surfaces, and deformation and the increase of the diameter corresponding to this change can be obtained to permit a strong grip on the bobbin. For this reason, the thickness of the flexible ring 16 according to the prior art becomes as great as 7 to 10 mm, and flexible rings having a smaller thickness cannot be used practically because deformation and the diameter increase becomes smaller and the bobbins cannot be firmly gripped.
On the other hand, when grip of the bobbin 23 is released, compressed air is supplied from a compressed air source, not shown, through the feed ports 12a, 14a so as to drive the piston 19 to the left in the drawing against the push force of the coned disc spring 18 and at the same time, the front cover 17 is moved to the left in the drawing to remove swelling of the rubber ring 21 due to deformation and the diameter increase and to release the bobbin 23.
According to the prior art described above, however, it is extremely difficult physically to provide a bobbin holder satisfying higher performance required in the future, that is, a bobbin holder capable of gripping a large number of bobbins having a small diameter and taking up a yarn at a high speed.
Next, the reason why the increased diameter of the bobbin holder 10 of the take-up device is essentially necessary to accomplish high speed take-up of the yarn in the prior art will be explained with a theoretical formula.
The constituent element that affects most greatly the determination of the specification of the bobbin holder 10 is the rotary cylinder 15 having the greatest length among the constituent elements, and its critical speed Nc, or in other words, its natural frequency, generally expressed by the following formula 1. The greater this value, the higher becomes the critical speed of the bobbin holder as a whole. ##EQU1##
Here, the sectional area A and the second moment of area I of the rotary cylinder have the following relation with the outer diameter Ds and the inner diameter Di of the rotary cylinder, respectively: EQU A.alpha.(Ds.sup.2 -Di.sup.2), I.alpha.(Ds.sup.4 -Di.sup.4 )
Accordingly, the formula 1 can be changed as follows: ##EQU2##
Here, symbols E and .rho. represent a Young's modulus and a specific gravity, respectively, and they are determined by the constituent material of the rotary cylinder 15. In the case of iron, they are generally 21,000 kg/mm.sup.2 and 7.85, respectively, and it is not greatly expected at present that the critical speed Nc can be increased to a great value due to these values.
Accordingly, in order to improve the critical speed, the following means must be employed:
1 to reduce the length L of the rotary cylinder; PA1 2 to increase the outer diameter Ds; or PA1 3 to increase the value of the inner diameter Di as much as possible within the allowable range permitted by the strength of the high speed rotary body. PA1 (A) a spring member exhibiting substantially a ring-like shape and shaped in such a manner that the sectional shape thereof in the direction of the driving shaft is convexed in at least the direction of the outer diameter; and PA1 (B) rigid rings disposed on both side surfaces of the spring member.
(1) As to the item 1, however, the length L is a value which is substantially determined by the full length of the bobbin 23 to be gripped, though it is designed to be as short as possible so as to improve the critical speed of the rotary cylinder 15. When eight bobbins each having a length L of 150 mm are gripped, for example, the length of the rotary cylinder 15 must essentially be a value approximate to the total length of the bobbins, i.e. 1,200 mm. Therefore, the length of from about 1,150 to about 1,200 is generally used, and this value cannot be reduced physically.
(2) As to the item 2, the diameters of the flexible rings 16a to 16h to be loosely fitted must he generally increased in order to increase the outer diameter Ds. This means essentially that the bobbins used must have a greater diameter, too. However, the increase in the diameter of the bobbins directly increases the cost of the bobbins, and the effects brought forth by the higher take-up speed and the improvement of productivity due to the increase of the critical speed (Nc) are offset by the increase of the bobbin cost.
Here, one effective means would be reducing as much as possible the thickness T of each flexible ring 16a to 16h in the radial direction and increases the outer diameter of the rotary cylinder 15. In other words, if the thickness of the flexible ring can be reduced without changing the inner diameter Db of the bobbins used, the outer diameter Ds of the rotary cylinder can be increased a corresponding amount, the critical speed of the bobbin holder can be increased and rotation at a higher speed becomes possible.
However, when the thickness T of each flexible ring 16a to 16h in the radial direction is merely reduced, the following problems occur. When eight bobbins 23 are held in the axial direction, for example, two flexible rings 16a to 16h are generally used for each bobbin and sixteen flexible rings, in total, are used. Each bobbin 23 is gripped by deformation of the flexible rings 16a to 16h in the direction of the outer diameter due to the push force from the front cover 17 as described above. When the thickness T is reduced on the basis of the concept described above, the total volume of the rubber ring 21 decreases and expansion in the direction of the outer diameter, capable of firmly gripping the bobbins 23 cannot be obtained even when the push force from the side surface direction acts. When the rubber ring 21 is merely elongated in the axial direction so as to compensate for the decrease of the volume resulting from the decrease of the thickness, the rubber ring 21 becomes a cylinder having a small thickness. Accordingly, it undergoes buckling when the push force acts on it and cannot expand in the direction of the outer diameter sufficiently for firmly gripping the bobbins 23.
On the other hand, the reduction of the thickness T of the flexible ring invites the problem in bobbin release. Even when the push force of the front cover 17 is released, the restoring force of the rubber ring 21 becomes small due to the reduction of the thickness of the flexible rings 16a to 16h. Though the rubber rings 21 of one or two flexible rings 16a, 16b near the front cover 17 return to their original shape, the diameter of the flexible rings 16g, 16h, etc, remote from the front cover 17 remains expanded because they cannot overcome the frictional force between the inner peripheral surface of the bobbins and the outer peripheral surface of the rotary cylinder 15, so that faulty bobbin removal occurs. As described above, the reduction of the thickness T of the flexible rings 16a to 16h in the radial direction so as to improve the critical speed of the rotary cylinder 15 to a high speed range cannot be accomplished as long as the prior art technology is employed.
Therefore, several prior art technologies are known as technologies which give specific design considerations to such a flexible ring. For example, U.S. Pat. No. 5,217,175 changes the sectional shape of the rubber ring that constitutes the flexible ring.
However, because the flexible ring relies only on the restoring force of the flexible material such as a rubber for the force of gripping the bobbin from the inner peripheral surface side, a sufficient gripping force cannot be obtained when the thickness is reduced. Further, the rubber ring in the means described above is shaped into the size of the outer diameter for gripping the bobbin under the state in which the stress from the direction of the side surface does not act, and when the bobbins are pulled out and removed, a tensile force is allowed to act on both side surface of the flexible ring so as to reduce the diameter and to cause deformation. In other words, since the bobbins are gripped under the state where any external force does not act, faulty bobbin gripping are likely to occur due to buckling of the rubber in the course of use for a long time and wear of its outer peripheral surface.
In the bobbin holder described in U.S. Pat. No. 3,923,261, the bobbin gripping portion is made only of a rubber material and applies the tensile force at the time of pull-out of the bobbins. Therefore, this Prior art involves the same problem as that of the flexible ring described above.
Accordingly, means for accomplishing the reduction of the thickness of the flexible ring, that is, means for increasing the outer diameter Ds of the rotary cylinder 15 without changing the inner diameter Db of the bobbins used, has not been accomplished at the present moment.
(3) Finally, in order to increase the inner diameter Di of the rotary cylinder 15 described in the item 3, the thickness t may be reduced, but when the thickness is excessively reduced, high-precision machining cannot be conducted and unbalance is likely to occur, so that excessive vibration is likely to occur at the time of high speed rotation. The decrease of the thickness t decreases the mechanical strength. Therefore, the flexible ring cannot withstand the centrifugal force at the high rotating speed, the possibility of mechanical damage exists, and destruction of the rotary cylinder might occur in the worst case.
In the case of a rotary cylinder having a full length of 1,150 mm, for gripping eight bobbins having an inner diameter of 94 mm, for example, the thickness of the rotary cylinder is set to about 4 to about 5 mm in order to insure precision of machining, at present. However, the reduction of the thickness to such a level cannot accomplish the improvement of the critical speed which can cope with the future need (refer to Japanese Patent Laid-Open No. 196268/1987, for example).
As described in the items (1) to (3), the prior art technologies have not been able to find any effective means for providing a bobbin holder for taking up the yarn at a high speed under the state where a large number of bobbins having a small diameter are held, by increasing the critical speed Nc of the rotary cylinder 15 to a high speed range without increasing the inner diameter of the bobbins.